Power Tip 57: Design a flyback primary switch snubber

Here's how to best control the voltage stress on the primary switch in a single-ended flyback converter.

How to best control the voltage stress on the primary switch in a single-ended flyback converter (shown in Figure 1) is a multi-faceted problem. You have to solve a combination of technical issues while still keeping an eye on the overall cost. You have to:

Limit the MOSFET voltage stress to an acceptable level

Discharge the leakage inductance very quickly to maintain good efficiency (see Power Tip 17)

Minimize circuit losses due to adding the snubber

Avoid impacting the power supply dynamics

The lowest cost approach to solve these issues is shown in Figure 1 of Power Tip 17 and consists of a standard recovery diode, capacitor and loading resistor. The circuit works by transferring excessive transformer leakage energy onto the snubber capacitor and dissipating it over the switching period. Unfortunately in this approach there is always energy dissipated in the snubber resistor, regardless of output power. In each switching cycle, the voltage on the capacitor always will be recharged to at least the reflected output voltage. This degrades the efficiency, particularly, at light loads.

Figure 1 of this power tip presents an alternative circuit approach, which replaces the resistor/capacitor with a resistor (R1) and zener diode (D1). When the FET turns off, the drain voltage rises to the point that the diodes conduct to discharge the leakage inductance of the transformer. The rate at which the current discharges is set by the difference between the reflected output voltage and the clamp voltage. Note that for best efficiency, as Power Tip 17 points out, it is critical to discharge the leakage inductance energy as quickly as possible. In choosing values, first consider the MOSFET voltage rating and derating criterion to determine a suitable maximum voltage stress on the MOSFET. First choose the zener voltage to be above the reflected output voltage so that it does not continue to conduct after the leakage inductance has been reset. Next size the resistor/zener combination so that you do not exceed the allowed MOSFET voltage stress at high-line and maximum current.

Click on image to enlarge.

Figure 1: This FET clamp provides good light load efficiency.

Now trade circuit ringing for efficiency. In Figure 2, resistor R1 has been shorted so that the zener solely sets the voltage stress on the MOSFET. At turn-off, the drain voltage flies up and the leakage inductance current is discharged with a constant voltage, which provides the fastest discharge and best efficiency. However, once the leakage inductance is discharged, the drain voltage rings around the reflected output plus input voltage, which creates a couple of concerns. Obviously one concern is electromagnetic interference (EMI), as this 4 MHz ringing creates common-mode currents in the power transformer and increases the power line filtering need. The second issue is related to the choice of controllers. There are a number of integrated circuits (ICs) that eliminate secondary-side measurement of the output voltage and rely on the primary bias winding voltage to provide a representative sample of the output. With this type of controller, the ringing can result in poor output voltage regulation accuracy.

If the ringing is an issue, reduce the zener voltage to approximately the reflected output voltage and add series resistance to increase the peak drain voltage. Figure 3 shows the waveforms from the circuit shown in Figure 1. The yellow trace is the drain voltage and the red is the voltage at the junction of D3 and R1. The difference between the two voltages is proportional to the leakage inductor current. The drain voltage starts at a high voltage and reduces the differential voltage and, hence, leakage inductance current to zero. So when the diode turns off, there is little voltage difference between the drain voltage and the reflected output voltage. Consequently, there is little ringing. Unfortunately, with this approach, you pay an efficiency penalty. In this case it was about two percent. As was pointed out in Power Tip 17, the longer it takes to discharge the leakage inductance, the worse the efficiency will be. In Figure 2, the leakage was discharged in 70 nS while it took 160 nS in Figure 3.

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Figure 3: Series resistance reduces EMI.

To summarize, RCD clamps are the simplest way to snub a flyback. However, with an RCD clamp, the light-load losses suffer from continuous power dissipation. If light-load loss is an issue, consider a snubber with a Zener, which only dissipates power when it is needed. An abrupt zener provides the best efficiency; but it can cause unacceptable ringing. The best trade-off may be using a reduced zener voltage along with a series resistance.

Please join us next month when we take a look at some classic power supply layout mistakes.

Robert Kollman is a senior applications manager and distinguished member of technical staff at Texas Instruments. He has more than 30 years of experience in the power electronics business and has designed magnetics for power electronics ranging from sub-watt to sub-megawatt with operating frequencies into the megahertz range. Robert earned a BSEE from Texas A&M University and an MSEE from Southern Methodist University.